Many nuclear receptor proteins act in trans to repress transcriptional responses to signaling pathways as a central aspect of their biological functions, but the underlying mechanisms remain an important and unresolved question in development and homeostasis.
The peroxisome proliferator-activated receptor γ (PPARγ) plays essential roles in fat cell development and glucose homeostasis. Further, PPARγ is the molecular target of insulin-sensitizing drugs, including rosiglitazone and other thiazolidinediones (TZDs)2,3 that improve insulin resistance by regulating the actions of PPARγ in several tissues4-8. PPARγ is thought to improve insulin resistance by both positively and negatively regulating gene expression, with PPARγ agonists suppressing the expression of inflammatory genes in adipocytes and adipose tissue-associated macrophages that are correlated with impaired insulin signaling9-11. PPARγ agonists have also been demonstrated to inhibit the development of atherosclerosis in animal models and to reduce the expression of inflammatory mediators within atherosclerotic lesions12-16. Gene expression profiling experiments suggest that transrepression is the primary transcriptional function of PPARγ in macrophages17. Although the ability of PPARγ and other nuclear receptor proteins to negatively regulate inflammatory gene expression by antagonizing the actions of NF-κB and AP-1 has been extensively studied18-26 (and reviewed in27), the molecular mechanisms remain poorly understood. Transrepression by PPARγ requires both its ligand binding and DNA binding domains, but not sequence-specific DNA recognition28. Consistent with this, genes that are subject to transrepression do not typically contain consensus recognition sites for PPAR/RXR heterodimers that mediate ligand-dependent activation of positively regulated genes. Furthermore, heterodimerization with RXR does not appear to be required, because PPARγ retains transrepression activity in macrophages lacking RXRα, the major RXR isoform in these cells.
The nuclear receptor corepressor, NCoR, and the related factor, SMRT, are components of corepressor complexes containing HDAC3, TBL1 and TBLR1 that interact with a subset of unliganded nuclear receptor proteins and mediate active transcriptional repression29-35. The TBL1 and TBLR1 components contribute to stability of binding to chromatin through histone interactions, while the HDAC3 component contributes to active repression35. Binding of nuclear receptor agonists results in the exchange of NCoR/SMRT complexes for coactivator complexes and a consequent switch in receptor function from transcriptional repression to activation32-34. TBLR1 also appears to play critical roles in the dismissal of NCoR complexes from nuclear receptor target genes by recruiting a Ubc5-containing ubiquitylation/19S proteosome complex36. Recent studies have extended the biological roles of NCoR/HDAC3/TBL complexes to NF-κB and AP-1 target genes, where they function to maintain inflammatory-response genes in a repressed state in the absence of inductive signals36-38.
The findings herein reveal a previously unrecognized molecular mechanism by which ligands regulate the transcriptional activities of nuclear receptors. This mechanism provides the basis for new screening strategies for the identification of novel classes of ligands that more effectively or more selectively activate anti-inflammatory actions of PPARs and other nuclear receptors. Such compounds would potentially represent improvements over existing drugs that exhibit significant side effects and/or suboptimal efficacy in inflammatory disease states including atherosclerosis and type II diabetes.